1. Field of the Invention
The present invention relates to a sealing gasket, especially for a flanged coupling.
2. Description of the Prior Art
In order to ensure leak-tightness of pipe-couplings, for example, in which thermal shock is liable to arise, expanded graphite is being put to increasingly widespread use and is progressively being substituted for asbestos both for economical, industrial and health reasons.
Patent No. FR-A-2 517 789 relates to a sealing gasket having an annular metallic core provided on each of its opposite faces with at least one circular groove or recess which is spaced at a distance from its central orifice and in which is placed a packing of expanded graphite. The packing projects above the metallic core so that, after clamping by means of suitable flanges, the expanded graphite is flush with the metallic core.
In some cases, the metallic core is formed by an inner ring and an outer ring, the opposite ring faces being cut in such a manner as to form in each case a V-section groove which is flared outwards.
One of the disadvantages lies in the absence of any stable metallic connection between the pipe-coupling flanges and each of the outer and inner rings owing to the fact that the flanges are in contact with the core of expanded graphite.
While the metallic rings are being brought together by clamping, that portion of the graphite which projects with respect to the rings is flattened by compression and induces within the graphite a nonuniform pressure field which tends to deform the inner ring in an unpredictable manner, to move it away from the outer ring and to enlarge this latter, these movements being neither prevented nor limited by any external obstacle.
By reason of the fact that the spacing between the two metallic rings increases during the clamping operation, there accordingly results a reduction in density of the expanded graphite core located between said rings. However, a reduction in density has an adverse effect on the sealing gasket while it is in use.
Patent No. FR-A-2 573 837 relates to a ring-shaped gasket having an annular core of molded expanded graphite between an inner metallic ring and an outer metallic ring, each ring being capable of acting as a spring in a direction parallel to the axis of revolution of the gasket. These rings can be formed in a number of turns wound in a tight spiral.
At the time of clamping of the gasket, the wavy surfaces of the walls become deformed and permit compression of the graphite core, with the result that the top and bottom faces of the core are at the same level as the metallic rings both before and after clamping of the gasket.
One of the drawbacks of a gasket of this type is irregular damping of the elastic recovery characteristics of the gasket as a result of friction between adjacent turns. This has a detrimental effect on the regeneration of contact pressures between the graphite and the coupling flanges during pressure and temperature changes.
Another disadvantage is the confinement of air and moisture between the metallic turns which tends to cause corrosion.
A further disadvantage lies in the fact that, as in the case mentioned earlier, it proves impossible to re-use a pair of metallic rings (usually the most costly part of the gasket). The user himself is quite unable to replace a graphite core which may have sustained damage during assembly on account of its fragility and which may not have suitable physical parameters, weight and density in compliance with the state of the joint or connection in which the gasket is intended to be employed.
Patent No. FR 86 10 121 relates to a gasket comprising at least one pair of annular metallic sheaths surrounding one sealing face of the gasket. The two annular metallic sheathing elements, namely the inner and outer elements, each have a cross-section in the form of an angle-iron member placed at one corner of the gasket. Each element has an annular cylindrical portion, the external surface of which forms a portion of the lateral surface of the gasket and has a height ranging in value from 25 to 50% of the thickness of the gasket, and an annular transverse portion, the external surface of which is inclined at each point of its transverse cross-sections at an angle of 90.degree. to 120.degree. with respect to the longitudinal direction of its annular cylindrical portion and surrounds said sealing face, the width of said portion being within the range of 10 to 45% of the width of the gasket.
This device is subject to the disadvantage of increasing the number of non-leaktight paths which are in communication with the fluid and of forming opposite to each other and at a short distance paths which are ready to evacuate leakages originating from the first paths.
As in the document cited earlier, zones between the confined metallic surfaces which are conducive to corrosion are formed.
The object of the present invention is to overcome the disadvantages mentioned earlier and to propose a sealing gasket having practical performances which are more effectively controlled, in particular in the presence of assemblies having large diameters and designed for high pressures.